1. Field of the Invention
This invention relates to an integrated process for thermochemically transforming biomass directly into fractionated and upgraded liquid fuels, particularly hydrocarbon fuels, such as gasoline and diesel boiling-point range materials, for example.
2. Description of Related Art
Conventional pyrolysis of biomass, typically fast pyrolysis, does not utilize or require H2 or catalysts and produces a dense, acidic, reactive liquid product that contains water, oils, and char formed during the process. In typical pyrolysis processing, char and ash are intermingled or intermixed. Therefore, hereafter references to char are to be understood as referring to a material that includes or may include char and intermingled or intermixed ash. Because fast pyrolysis is most typically carried out in an inert atmosphere, much of the oxygen present in biomass is carried over into the oils produced in pyrolysis, which increases their chemical reactivity. The unstable liquids produced by conventional pyrolysis tend to thicken over time and can also react to a point where hydrophilic and hydrophobic phases form. Dilution of pyrolysis liquids with methanol or other alcohols has been shown to reduce the activity and viscosity of the oils, but this approach is not considered to be practical or economically viable, because large amounts of unrecoverable alcohol would be required to stabilize and transport large quantities of pyrolysis liquids.
In conventional pyrolysis carried out in an inert environment, the water miscible acidic liquid product is highly oxygenated and reactive. Conventional pyrolysis oils are characterized by total acid numbers (TAN) in the range of 100-200, low chemical stability for polymerization, incompatibility with petroleum hydrocarbons due to water miscibility, very high oxygen content, on the order of about 40% by weight, and a low heating value. As a result, the stabilization, transportation, and utilization of pyrolysis-derived liquids are problematic and it is difficult to upgrade this product to a liquid fuel due to the retrograde reactions that typically occur in conventional pyrolysis and in conventional fast pyrolysis. In addition, the separation of char generated by conventional pyrolysis from the liquid pyrolysis product presents a significant technical challenge due to the large amounts of oxygen and free radicals in the pyrolysis vapors which remain highly reactive in a vapor state and form a pitch-like material when they come in intimate contact with char particles on the surface of a barrier filter. Consequently, filters used to separate the char from the hot pyrolysis vapors tend to blind quickly due to the reactions of char and unstable oils that occur on and within the layer of separated char on the surface of the filter.
The upgrading of pyrolysis oils produced by conventional fast pyrolysis via hydroconversion consumes large quantities of H2 and the extreme process conditions make it uneconomical. Also, the reactions in such processing are inherently out of balance in that, due to the high pressures required, more water is formed than the process requires while more H2 is consumed than is produced by the process. This leads, in part, to a requirement for an external source of H2. In a balanced process, all the hydrogen required by the process is produced by the process and water produced by the process is in large part consumed. In addition, when upgrading conventional pyrolysis oil, hydroconversion reactors often plug due to coke precursors present in the pyrolysis oils or from coke produced as a result of the catalytic hydroconversion process.
In general, hydropyrolysis is a catalytic pyrolysis process carried out in the presence of molecular hydrogen. Hydropyrolysis may be an unfortunate name in that it could be taken to be an aqueous process. However, for those skilled in the art, the process context provides sufficient clarity to avoid such misconception. Typically, the objective of conventional hydropyrolysis processes has been to maximize liquid yield in a single step. However, in one known case, a second stage reaction was added, the objective of which was to maximize hydrocarbon yield while maintaining high oxygen removal. However, even this approach compromises economy, because excessive internal pressures are required along with an external source of H2.
Because of such inefficiencies, significant interest remains in the economical production of hydrocarbon fuels from biomass, particularly, gasoline and diesel boiling-point range materials.